Introduced in 2003 by Directive 2003/87/EC, the greenhouse gas emissions trading system (ETS) is a political instrument in the fight against climate change and an essential tool for reducing greenhouse gas emissions. Industrial operators included in the system according to the rules set by the directive must monitor, declare and have their emissions verified in order to restitute a number of allowances in the EU ETS registry that is equal to their CO2 emissions. To do this, they can receive emission allowances for free, buy allowances or sell allowances.